After lots of trial and error, I was finally able to get the multiple "About Author" boxes to work at the end of posts, mostly thanks to the instructions in this very helpful thread.
It works perfectly with regular WP authors, but I just discovered that the link to author pages doesn't work for guest authors. I've searched through the past 10 pages of support topics and couldn't find anything.
Here is the code I'm using for the "About Author" boxes:
Any ideas? Thanks in advance! This is a fantastic plugin.